We started to learn about the rapid deterioration of instrument skills and knowledge as we worked on the rating. If we laid off from training for more than a week, we found that we had to do more than a little review during the next lesson to unfog our brains on procedures and focus extra carefully to hold altitude, heading and speed within the parameters set by that unreasonably demanding, evil CFII in the right seat.
Once we had the rating it was unpleasant to discover just how fast the rust could set in. There were times that we launched with a safety pilot, put on the foggles and immediately put the airplane through a some gyrations while suffering a bit of spatial disorientation before we could settle down and keep the airplane going where we wanted.